1. Field of the Invention
The present invention relates in general to a control circuit for controlling an AC motor of an elevator system and, in particular, to a control circuit for use in an elevator system which utilizes an AC drive motor in conjunction with an eddy current brake.
2. Description of the Prior Art
In previous elevator control circuits for AC motors, phase control of SCR's was utilized to control the motor speed and the position of an associated elevator car. These circuits typically included a motoring gain amplifier responsive to a tachometer signal representing actual car speed and a pattern signal representing the desired car speed for generating a motoring gain signal to control the SCR's. The pattern signal generally consisted of an acceleration portion, a full speed portion, and a deceleration portion. In order to control the motor, a reduced voltage was applied to the motor during acceleration, and the SCR's were fully turned on for a full speed run.
In those systems, a relatively large flywheel was attached to the motor shaft to assist in providing smooth acceleration and deceleration of the associated elevator car. The flywheel provided sufficient inertia such that when the elevator car was traveling in a full load, hauling condition in the up direction, all the power could be removed from the motor at a certain slowdown point and the rotational energy of the flywheel was generally sufficient to move the car to the selected floor. In instances wherein the flywheel provided too much inertia, contactors connecting the motor to the power source would be reversed and power applied to the motor to provide braking of the car. The contactors to the motor would also be reversed in the event an overhauling condition was detected
One of the problems associated with AC systems occurs during the transition period when the control circuit is switching from the full speed portion into the deceleration portion of the pattern signal. In some instances, depending on different conditions such as the actual speed of the car, such a transition period produces a bump which is transmitted to the passengers in the car.